The invention relates to monitoring rotation of a pneumatic tire, with Telemetry apparatus disposed in the tire.
It is known to remotely monitor conditions of pneumatic tires of motor vehicles. For example, telemetry devices comprising an RF transmitter and one or more condition sensors may be disposed in each of the tires. A transponder and associated conditions sensors (e.g., pressure, temperature) may also be disposed in pneumatic tires of motor vehicles. A xe2x80x9ctransponderxe2x80x9d is an electronic device capable of both receiving and transmitting radio frequency (RF) signals. These transponders transmit a RF wave, with or without variable data (e.g., pressure, temperature) and/or fixed data (e.g., tire ID) to outside the tire, and receive RF signals, with or without data, from outside the tire. A separate transponder is typically associated with each tire of a motor vehicle to monitor and transmit tire-related data. Typically, a single xe2x80x9cinterrogatorxe2x80x9d having both transmitting and receiving capabilities is used to communicate with the plurality of transponders. The interrogator may be xe2x80x9chand-heldxe2x80x9d, or mounted xe2x80x9con-boardxe2x80x9d the vehicle, or positioned along or in a roadway (e.g., xe2x80x9cdrive-overxe2x80x9d, or xe2x80x9cdrive byxe2x80x9d).
xe2x80x9cActivexe2x80x9d transponders have their own power supply (e.g., a battery). They transmit signals, and are typically also capable of receiving signals to control their functionality.
xe2x80x9cPassivexe2x80x9d transponders are powered by the energy of an incoming RF signal, such as from an interrogator. Passive transponders fall into two general categories, those having only passive circuitry, and those having some active circuitry. In the main, transponders which are passive transponders having some active circuitry are discussed herein.
Transponder systems including a plurality of transponders and a single interrogator are well known and disclosed, for example, in U.S. Pat. No. 5,339,073 (Dodd, et al.; 1994), incorporated in its entirety by reference herein, which shows that both the interrogator and the individual transponders may comprise microprocessors. An example of a device for monitoring, storing and telemetering information such as temperature, pressure, tire rotations and/or other operating conditions of a pneumatic tire, along with tire identification information, can be found in U.S. Pat. No. 5,573,611 (Koch, et al.; 1996), incorporated in its entirety by reference herein.
FIG. 1A illustrates a typical tire pressure monitoring system 100 of the prior art installed on a motor vehicle 102 (shown in dashed lines) having four pneumatic tires 104a . . . 104d installed on four respective wheels (not shown). A transponder (xe2x80x9cTAGxe2x80x9d) 106a . . . 106d is disposed within each of the tires 104a . . . 104d, respectively. The transponders 106a . . . 106d are preferably passive transponders which obtain their operating power from an RF signal such as is typically generated by an on-board interrogator 108 which is mounted within the vehicle. A sensor (not shown), such as a pressure sensor or a temperature sensor, is typically associated with each transponder 106a . . . 106d. 
The interrogator 108 comprises an RF transmitter 112 (e.g., for powering passive transponders), an RF receiver 114, control logic 116 which may include a microprocessor (xcexcP) , and a display device 118 such a visual display and optionally including an audible alarm. Antennas (xe2x80x9cANTxe2x80x9d) 110a . . . 110d are disposed on the vehicle 102, preferably adjacent the tires 104a . . . 104d, respectively, such as in the wheel wells of the vehicle. The antennas 110a . . . 110d are suitably ferrite loopstick antennas.
The use of multiple vehicle antennas 110a . . . 110d, each at a fixed position on the vehicle adjacent a respective tire 104a . . . 104d is well known and preferred, and is disclosed in U.S. Pat. Nos. 3,553,060; 3,810,090; 4,220,907; 5,541,574; and 5,774,047, all of which are incorporated in their entirety by reference herein.
In use, the interrogator 108 powers the transponders 106a . . . 106d which, in turn, transmit data indicative of a measured condition (e.g., air pressure) back to the interrogator. In any such system, it is desirable to have efficient and effective coupling of signals between the fixed antennas 110a . . . 110d (alternatively, one centrally-located fixed antenna) and the moving (i.e., when the vehicle is in motion) transponders (each of which has its own antenna, not shown).
FIG. 1B illustrates, generally, the physical arrangement of a transponder 106 (representative of any one of the transponders 106a . . . 106d) disposed within a pneumatic tire 104 (representative of any one of the tires 104a . . . 104d). The transponder 106 is mounted in any suitable manner to an inner surface 105 of the tire 104. An antenna 110 (representative of any one of the antennas 110a . . . 110d) is disposed on the vehicle near the tire 104 such as in a wheel well, to provide electromagnetic radiation to power the transponder 106 (in the case of a passive transponder) , as well as to receive signals from the transponder 106.
In many industries there exists a need for a device capable of automatically counting revolutions of a rotating element, storing a data count, and transferring the data to a computing device for manipulation. The degree of accuracy (or resolution, required varies from a count of complete revolutions or rotations (low resolution) to a measurement of partial revolutions in very small increments (high resolution). Monitoring revolutions of vehicle tires is of particular interest with regard to the present invention.
U.S. Pat. No. 3,613,075 (Griffiths; 1971), incorporated In its entirety by reference herein, discloses monitoring tire inflation by sensing the angular movement of a wheel by counting revolutions of the wheel and comparing the count with a count from another wheel or a reference to determine whether or not the wheel has the proper diameter (hence, proper inflation). For counting revolutions of a wheel, a block of metal is mounted for rotation on the brake drum of the wheel and projects radially outwardly to pass in close proximity to a pickup which may be a normally open switch which is held closed by the attraction of a permanent magnet forming a part of the switch mechanism. Counting circuits are disclosed in this patent. See also U.S. Pat. No. 3,691,524 (Frost, et al. 1972).
U.S. Pat. No. 5,524,034 (Srygley, et al.; 1996), incorporated in its entirety by reference herein, discloses an automatic revolution counting and data transmission device in the form of a hub-odometer for measuring rotations of a wheel on a vehicle. The use of reed switches (9) and a magnet (14) for detecting rotation is described. The switches provide signals which relate directly to the revolution of the hub unit. These signals are processed into a suitable machine-readable format for use by a microcontroller (7) As noted in the patent (column 3, lines 28-30), xe2x80x9cthe signal must be lengthened, de-bounced, and output at the correct signal level. The signal conditioner (8) takes care of this task.xe2x80x9d The possibility of using optical sensors, gear tooth sensors, Hall effect devices, and contact sensors in lieu of the reed switches and magnets for sensing rotation is disclosed.
U.S. Pat. No. 4,862,486 (Wing, et al.; 1989), incorporated in its entirety by reference herein, discloses a revolution counter that is attached to the vehicle tire. As aptly noted in this patent, a problem with mechanical counters attached to the axle or wheel hub is that by not being mounted to the tire itself, the counter must be changed or separate records must be maintained whenever the tire is changed to maintain a proper count for a given tire. This patent describes revolution-counting apparatus comprising a piezoelectric polymer sensor (5) which senses a change in stress as a given section of a tire is stressed with each revolution. The output of the sensor (5) is conditioned by a Schmidt trigger (15) or other suitable circuit which is effective in shaping and amplifying the electrical pulse from the sensor so that it is suitable as an input to the counter (13). The counter circuit is preferentially powered by a battery (9), and it is preferred to electronically download the information of the counter at such time as tire failure has occurred (see column 2, lines 5-35).
Piezoelectricity, literally xe2x80x9cpressure electricityxe2x80x9d is a property of certain materials such as quartz, Rochelle salt, and certain solid-solution ceramic materials such as lead zirconate-titanate (Pb(Zrl-xTix)03) (xe2x80x9cPZTxe2x80x9d). In a xe2x80x9cgeneratorxe2x80x9d mode, electricity is developed when a piezoelectric (xe2x80x9cpiezoxe2x80x9d) crystal is mechanically stressed. Conversely, in a xe2x80x9cmotorxe2x80x9d mode, the piezo crystal reacts mechanically when an electric field is applied. Applications for piezo crystals include force transducers, spark pumps for cigarette lighters and boiler ignition, microphone heads, stereophonic pick-ups, etc.
PZT is one of the leading piezoelectric materials used today. It can be fabricated in bimorph or unimorph structures (piezo elements), and operated in flexure mode. These structures have the ability to generate high electrical output from a source of low mechanical impedance (conversely, to develop large displacement at low levels of electrical excitation).
U.S. Pat. No. 3,456,134 (Ko; 1969), incorporated in its entirety by reference herein, discloses a piezoelectric energy converter for electronic implants, wherein body motion is converted into electrical energy using a piece of piezoelectric PZT in the form of a resonant cantilever beam. The piezoelectric beam is enclosed in a small case whose free end is loaded with a weight to resonate at a suitable frequency corresponding to the movement that drives the enclosing case. As the base of the case is moved periodically, the PZT wafer vibrates at its natural frequency. The electrical pulse is generated by the vibration and then is rectified by a voltage doubler to store in a capacitor. The converted electrical energy is used to supply the implant device or pacemaker. When the PZT converter is implanted on the surface of a dog""s heart, the electrical output is estimated to have generated 4.0 volts at 105 ohms, or 160 microwatts.
U.S. Pat. No. 4,510,484 (Snyder; 1985), incorporated in its entirety by reference herein, discloses a piezoelectric reed power supply for use in abnormal tire condition warning systems. A device is provided for sensing a tire condition, and is mounted on a tire rim so as to be subject to vibrations normally occurring as a result of operating the vehicle. The power supply includes a piezoelectric reed having a base portion and an end portion. A tuning mass member is mounted to the end portion. The tuning mass is sized relative to the piezoelectric reed to obtain a resonant frequency of vibration of the power supply induced by common road vibrations during wheel operation. Various mechanisms for limiting flexure of the piezoelectric reed and inhibiting compound bending of the reed are disclosed.
U.S. Pat. No. 4,504,761 (Triplett; 1985), incorporated in its entirety by reference herein, discloses a vehicular mounted piezoelectric generator, and is incorporated in its entirety by reference herein. A piezoelectric array is mounted on one or more tires of a motor vehicle. As the vehicle drives down the road, the tire is flexed during each revolution to distort the piezoelectric elements and generate electricity. An electric circuit delivers the energy to the electrical system of the vehicle.
Piezo elements are known to be used in conjunction with telemetry systems, such as wheel-mounted telemetry systems for monitoring the conditions of pneumatic vehicle tires.
U.S. Pat. No. 4,237,728 (Betts, et al.; 1979) incorporated in its entirety by reference herein, discloses a low-tire warning system capable of detecting under-inflation of a pneumatic tire when the tire is rotating, and is incorporated in its entirety by reference herein. Each tire has a telemetry unit with a piezoelectric transducer which is adapted to be mounted adjacent a pneumatic tire on a vehicle, and deflected with each revolution of the tire when the tire profile is low. Pulses from the transducer are accumulated on a capacitor to provide a power supply. Upon reaching a threshold charge, a counter commences counting the pulses.
When the counter reaches a predetermined count, it produces an enable signal. An encoder produces an encoded signal. A transmitter for transmitting a modulated radio signal receives the encoded signal for modulating a carrier frequency to produce a modulated radio signal. At least one of the encoder and transmitters are connected to the counter to be enabled by the enable signal.
Attention is also directed to the following, each of which is incorporated in its entirety by reference herein: U.S. Pat. No. 5,260,683 (Tanaka, et al.; 1993; a piezo element is deformed by tire pressure); and U.S. Pat. No. 5,581,023 (Handfield, et al.; 1996; pressure transducer including a piezo-resistive, variably-conductive layer).
Many crystals have been found to possess piezoelectric property. Until the late 1940""s piezoelectric materials included quartz, Rochelle salt, tourmaline, ammonium dihydrogen phosphate (ADP), and lithium sulfate monohydrate. Not only single crystal, but polycrystalline ceramics solid can present piezoelectricity after subjected to a xe2x80x9cpolingxe2x80x9d field. Since 1957 lead zirconate-titanate (PZT) solid-solution ceramics has become one of the most important piezoelectric materials which offers high piezoelectric coupling, wide operating temperature range, and a choice of useful variations in engineering parameter.
Piezoelectric ceramics are generally made by a solid state reaction of several oxides or carbonates, followed by high temperature firing involving crystal grain growth, and the electric poling process. Most piezoelectric ceramics are solid solutions. Variation of chemical composition allows the optimizing of properties. The leading position of the PZT compositions is due to their intrinsically strong piezoelectric effect and high Curie point, which allow a wide variation in chemical composition to obtain a wide range of operating parameters without serious reduction of the piezoelectric effect. By changing the composition of titanium and zirconium, the properties of the PZT change. There are many types of PZT, including Clevite PZT-2, PZT-4, PZT-5, PZT-6, PZT-7, PZT-8 etc. PZT-2 has rich zirconium composition, exhibiting high coupling property. PZT-4 has some calcium, strontium or barium to replace lead, and tin for zirconium, resulting in lowered Curie point and increased permittivity. The PZT-5 is electron-donor doped lead zirconate-titanate, using a high valence composition to replace the low valence composition, niobium (valence 5) for titanium (valence 4) or lanthanum (valence 3) for lead (valence 2). The resulting PZT-5 has an enhanced permittivity and compliance, increased dc resistivity, and reduced aging rate. PZT-6 shows long time stability, PZT-7 exhibits low permittivity, and PZT-8 shows low dielectric loss at high electric drive.
Applications of piezoelectric materials can generally be divided into two classes: resonant and non-resonant. The present invention is directed principally to non-resonant applications of piezo elements, such as applying mechanical stress thereto by bending the piezo element.
FIG. 2A is a cross-sectional view of a xe2x80x9cbimorphxe2x80x9d piezo element 200. A xe2x80x9cbimorphxe2x80x9d is a flexing-type piezoelectric element, which has the capacity for handling larger motions and smaller forces than single piezoelectric plates. The bimorph 200 comprises two planar piezo crystals 204 and 208 secured together face-to-face with a shim or vane 206 therebetween. When a voltage is applied to the electrodes 202 and 210, the two crystals 204 and 208 are caused to deform in opposite directions, one in expanding mode and one in shrinking mode, which result in a bending action. Conversely, mechanical bending of the element 200 will cause it to develop a corresponding voltage between the electrodes 202 and 210. The bimorph 200 may have its two crystals 204 and 210 connected either in series or parallel.
FIG. 2B is a cross-sectional view of a xe2x80x9cunimorphxe2x80x9d piezo element 220. The unimorph 220 comprises a planar piezoelectric plate 224 (compare 204) bonded to a plate 226 (e.g., a brass plate) and having an electrode 222 (compare 202) on an opposite surface thereof. The operation of the unimorph 220 is essentially the same as that of the bimorph 200, since as the piezo crystal 224 attempts to react to an electrical signal, it is contained at the bonded surfacexe2x80x94 the net result being deflecting or bending. Conversely, flexing of the unimorph 220 will result in the generation of electrical energy by the piezo crystal 224.
There are a number of design challenges attendant incorporating piezo elements in tires, and implementing tire rotation counting schemes. A piezo element incorporated into a tire must be durable. It should also last a long time, such as 5-10 years. Any circuit being powered by a piezo element rather than a battery or by RF from an interrogator should consume very little power since the power is limited. Revolution data needs to be able to be increased continuously and stored permanently even with an interrupted power source. When required, the data should be able to communicate via wireless RF telemetry to an outside reader.
It is an object of the present invention to provide method and apparatus for monitoring one or more dynamic operating conditions of a rotary element such as a pneumatic motor vehicle tire, as defined in one or more of the appended claims and, as such, having the capability of being implemented in a manner to accomplish one or more of the subsidiary objects.
It is a further object of the invention to count the revolutions of a pneumatic tire utilizing a sensor which responds to the periodic mechanical stresses as the tire rotates under load on a load bearing surface such as a roadway.
It is a further object of the invention that the tire revolution counter is self powered, without requiring a battery or other stored energy device, and also not requiring an external source of power such as radiated RF energy.
It is a further object of the invention that the self powered tire revolution counter be mounted inside of a pneumatic tire and will continually update the total tire revolution count and store the same in a non-volatile storage medium within he tire for the lifetime of the tire.
It is a further object of the invention that the self powered tire revolution counter will make the updated total revolution count available to other reading devices, sensors, controllers and telemetry devices such as transponders which may also be mounted inside of the same pneumatic tire.
According to the invention, a self-powered revolution counter comprises a mechanical-electrical energy converter and a revolution counting circuit. One piezoelectric crystal element (piezo element) acts both as an energy converter and as a revolution sensor. The piezo element transforms mechanical energy into electrical energy and then serves as a power supply to the revolution counting circuit. The revolution counting circuit receives the revolution signals, processes them, increases and stores the new count into a non-volatile memory. The counting circuit can be programmed to convert the revolution count to an approximate mileage total. This self-powered revolution counter includes (1) a piezoelectric mechanical-electrical energy converter/power supply, (2) an analog circuit for signal conditioning and shaping, (3) a digital logic circuit for counting, and (4) a microcontroller with built-in EEPROM for non-volatile data storage.
According to an aspect of the invention, the piezo element as molded into the tire; it is deformed when the tire section with the piezo element contacts ground; and the induced stress generates an electrical pulse and damped oscillations via the piezoelectric effect. The generated pulses are rectified and conditioned to serve as a DC power source, and at the same time they are used as an indicator of tire revolutions. Therefore, one piezoelectric element can be used simultaneously for energy conversion and for counting tire revolutions when the vehicle is moving.